OBJ files: How to subdivide materials for model?
There are two ways you can do this, with Materials, or with the Texture. 
In Blender you can add materials and assign then where you select the mesh (Click Here to see a similar question) 

Or you can also keep only one material and customize the image texture that comes with the .obj When you download the model check out a small image that contains 4 colors, that's the image texture for the model. 

In case you have any questions about assigning texture in Blender check

This for Blender Internal

This for Cycles

 Image texture



Since the model already has a UV mapping, you only need a 2D Image Editor (Photoshop, GIMP, etc..) Then you can change the colors for your desired parts.



After saving that image, in Blender, on the UV Editor, click Image > Reload Image